Fix musl build

Patch written by Thomas Petazzoni:
http://patchwork.ozlabs.org/patch/572309/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>

diff -uNr mjpg-streamer-bbf32fddfd02a9e072e89e83a5b33e6ca0a7bd4b.org/plugins/input.h mjpg-streamer-bbf32fddfd02a9e072e89e83a5b33e6ca0a7bd4b/plugins/input.h
--- mjpg-streamer-bbf32fddfd02a9e072e89e83a5b33e6ca0a7bd4b.org/plugins/input.h	2015-11-09 15:56:47.000000000 +0100
+++ mjpg-streamer-bbf32fddfd02a9e072e89e83a5b33e6ca0a7bd4b/plugins/input.h	2016-01-26 20:19:05.884349502 +0100
@@ -20,6 +20,7 @@
 #                                                                              #
 *******************************************************************************/
 
+#include <pthread.h>
 #include <syslog.h>
 #include "../mjpg_streamer.h"
 #define INPUT_PLUGIN_PREFIX " i: "
